1. Field of Invention
The present invention relates to a polarization splitter. More particularly, the present invention relates to a structure and a method for fabricating a wide-angle polarization splitter combining a straight waveguide and a branch waveguide.
2. Description of Related Art
A polarization splitter in a waveguide typically is fabricated on a LiNbO.sub.3 crystal substrate. Generally, the polarization splitter is divided into two types. One type is called as a direction coupler type that is fabricated according to the modes with different polarization directions and their different coupling lengths. In this manner, the coupling length between two polarization modes needs to satisfy a special requirement in order to obtain the higher extinction ratio, and it therefore needs a small error tolerance in fabrication. Another type of polarization splitter is call a Y-branch type that has a Y-like structure with two different polarization properties, so that two polarization modes can be split into, such as TE and TM modes. The polarization splitter with the Y-branch type, for example, has been disclosed in U.S. Pat. No. 5, 436,992. For the conventional Y-like polarization splitter, if the branching angle is greater than 2 degrees, the propagation loss is very large. This is the reason why the branching angle of the conventional Y-like polarization splitter cannot be effectively reduced. Moreover, a conventional nickel diffusion manner for fabricating the Y-like polarization splitter needs high temperature and long diffusion time to obtain an ordinary polarized waveguide with a single polarization direction. This fabricating manner has a disadvantage with a poor optical confinement.
The fabrication method for the Y-like polarization splitter on the LiNbO.sub.3 crystal substrate typically includes three technologies to have an input end that is a random polarization waveguide, one output end that is an extraordinary polarization waveguide with a single polarization direction, and another output end is an ordinary polarization wave guide. The extraordinary polarization waveguide can be fabricated by manners of magnesium-oxide diffusion or proton exchange, and the ordinary polarization waveguide cab be fabricated by nickel diffusion. Due to the property of single polarization direction, the splitter has a high optical extinction ratio between, for example, the TE mode and the TM mode. However, the technologies above cannot fabricate a Y-like structure with a branching angle greater than 2 degrees. This also indicates that the dimension of the polarization splitter cannot be effectively reduced for the conventional technologies. The nickel diffusion manner needs high temperature and long diffusion time to obtain an ordinary polarized waveguide with a single polarization direction. Further still, the optical confinement for the waveguide is also poor.
Recently, some waveguide products have been developed to have a wide branching angle with good optical transmission, such as a prism type or a substrate prism type with a large branching angle, in which the substrate prism type needs only once of photolithography process. However this technology has not been applied to a polarization splitter yet.